Conventionally, a base substrate is subjected to a selective process by, e.g., etching or diffusion in the manufacture of a semiconductor integrated circuit element, a mask for manufacture of an integrated circuit, a printed wiring board, a printed board and the like. In this case, a composition which is sensitive to active rays of light such as ultraviolet rays, X-rays or electron rays, is used to form a so-called photoresist film on the base substrate in order to selectively protect non-process portions of the substrate. Light is then selectively directed onto the photoresist film employing a mask, and thereafter photoresist patterns are formed on the substrate by developing the photoresist film.
There are two types of the photoresist, namely positive type and negative type. The positive photoresist is of such a type that the exposed portion dissolves in a developer, while the unexposed portion does not dissolve therein, and the negative photoresist is of the opposite type. A representative of the positive photoresist is a composition of an alkali-soluble novolak resin being a base and a naphthoquinone di-azido compound being an agent for photodecomposition. For this quinone di-azido-type positive photoresist, a water solution of tetramethyl ammonium hydroxide of 2.38% by wt (weight) is generally employed as a developer.
FIGS. 2A-2E are cross-sectional views showing conventional processing steps of forming a photoresist pattern.
Referring to FIGS. 2A and 2B, a uniform thin film of a photoresist 1 is first formed on a substrate 2 by dropping a solution of the positive photoresist on substrate 2 and then spinning substrate 2. For the positive photoresist are employed MCPR2000H (manufactured by Mitsubishi Kasei Co., Ltd.) MCPR3000E (manufactured by Mitsubishi Kasei Co., Ltd.) and TSMRV5 (manufactured by Tokyo Ohka Co., Ltd.). These positive photoresists generally have an unexposed portion dissolution rate with a 2.38% aqueous solution of tetramethyl ammonium hydroxide of about 1 .ANG./sec or less. For example, MCPR2000H has an unexposed portion dissolution rate of 0.16 .ANG./sec, MCPR3000E has an unexposed portion dissolution rate of 0.10 .ANG./sec and TSMRV5 has an unexposed portion dissolution rate of 0.10 .ANG./sec. The dissolution rate of the unexposed portion of these resists with respect to the developing solution is significantly lower than that of conventional resists. The difference in dissolution rates for the developing solution between the exposed portions and the unexposed portions of the resists is increased in order to achieve a higher resolving power.
Referring to FIG. 2C, ultra-violet rays 4 are then selectively directed onto thin photoresist film 1 through a photomask 3 on which an electronic circuit is drawn.
The substrate 2 is then placed on a hot plate and heated at 100-120.degree. C., as shown in FIG. 2D. This heating process to be carried out after exposure and before development is called a PEB (Post Exposure Bake) process. This step is not indispensable but may be carried out if necessary.
Referring now to FIG. 2E, development of the resultant film with the water solution of tetramethyl ammonium hydroxide of 2.38% by wt causes dissolution of the portion irradiated with light, thereby forming a pattern of photoresist 1.
Next, a photosensitive mechanism of the positive photoresist is shown in FIG. 3. Naphthoquinone di-azido I! residing in the photoresist absorbs ultraviolet rays, then transforms to carbene II! and then to ketene III!. This ketene III! reacts with water existing in the system and then transforms to indene carboxylic acid IV!. This indene carboxylic acid IV! dissolves in an alkali solution of the developer, whereby the exposed portion of the photoresist 1 is removed, as shown in FIG. 2E.
A super LSI is currently manufactured according to a design rule of 1 .mu.m or less by employing the above-described positive photoresist materials.
The foregoing conventional method, however, causes the following disadvantages, as a degree of integration of super LSI becomes further enhanced, as described with reference to FIGS. 4A-4D.
A principle of the formation of a positive photoresist pattern is briefly the utilization of a difference in dissolution rates between the exposed portion and the unexposed portion of the photoresist.
Referring to FIG. 4A, ultraviolet rays 4 directed to the photoresist 1 produce interferences with light reflected from the substrate 2 in the film of the photoresist 1 and, consequently a standing wave occurs. Thus, the degree of decomposition of a photosensitive agent in the film of the photoresist 1 is uneven, thereby producing distributions as shown in the figure. Accordingly, development causes formation of a pattern in accordance with the distribution in degree of the decomposition of the photosensitive agent, as shown in FIG. 4B.
When the width of the pattern is extremely narrow (approximately 0.5 .mu.m), photoresist in the vicinity of the substrate 2 is not developed even by a further advanced development, and thus the photoresist 1 is not completely removed, with reference to FIG. 4C. Consequently, a resist pattern having a poor resolving power is obtained, as shown in FIG. 4D.
In order to solve these disadvantages, Japanese Patent Laying-Open No. 61-232454 discloses a technique in which a trialkyl methyl ammonium compound having an alkyl group containing 3 to 5 carbons is added to a developer. According to this conventional technique, the addition of the above-described quaternary ammonium compound to the developer enables a considerable degradation in the solubility of the exposed portion. This enhances the selectivity in dissolution between the exposed portion and the unexposed portion and also enhances the resolution.
This method, however, still has problems in causing a degradation in the solubility of the exposed portion as well as the unexposed portion, and consequently, a degradation in total sensitivity results.